It is often desirable, in a vehicle, that certain rotary devices, for example the radiator cooling fan, be driven at speeds different than engine speed. Driving a cooling fan in a one to one relationship with the engine drive shaft or with a shaft driven by the engine may yield a fan speed that is too high, and wasteful of energy, at higher engine speeds. Many variable speed fan drives are known and used in conjunction with radiator cooling fans. These provide fan slippage at high engine speeds, and are also selectively operable dependent upon radiator temperature. Such devices may be somewhat voluminous and complex, but are warranted by the large amount of energy to be saved.
Typically, the cooling fan for a vehicle alternator is rigidly mounted to, and turns one to one with, the alternator shaft. Being a much smaller fan, both cost and space considerations prevent the use of fan drives such as those found in radiators. However, it would also be desirable to drive an alternator cooling fan in a selectively operable fashion, if it could be done simply and economically. Energy could be saved, and the fan would not have to withstand the stresses caused at higher engine speeds by a rigid mounting. Additionally, the need for cooling in a typical vehicle alternator is not related to engine speed alone. The need for cooling in a vehicle alternator is most directly related to the load thereon, the current that it is called upon to produce. Therefore, it would be desirable to have a fan drive that was selectively operable, i.e. directly responsive to that need for cooling, as opposed to merely providing fan slippage at high shaft speeds.
The U.S. Pat. No. 4,437,846 to Ragaly discloses an alternator fan drive including a pair of axially side by side bearings interposed between the hub of the cooling fan and the generator shaft. The inner races of the bearings are rigid to the generator shaft. The outer races of the bearings are fitted within the hub of the fan, one rigidly fitted and one axially slidable therewithin. A spring biases these outer races apart and into the bearing balls, providing a preload. The cooling fan will turn one to one with the generator shaft at low shaft speeds, because the preload is not overcome. At higher shaft speeds, the preloaded bearings and the fan will slip relative to the shaft. While there is an energy savings over a rigid mounting, the drive means is responsive only to varying shaft speed. In addition, the structure is somewhat complex, requiring two bearings and a spring, as well as a consequent lengthening of the fan hub to include the bearings. A matching offset portion must also be added to the drive pulley to accommodate the fan hub and bearings.